1. Field of the Invention
The invention relates to the manufacture of optical fiber preforms and preform rods.
2. Information Disclosure Statement
Silica activated lightguides with flourine doped silica (SiO2—F) claddings and cores doped with Al or Ge and/or Rare earth elements (Neodymium, Erbium, Holmium, Ytterbium, Europium and so on) are widely used for the creation of optical fiber amplifiers, lasers and sensors of different types. In UV fibers, however, the presence of Germanium can result in ultraviolet radiation-induced color centers, which are point defects in the core's silica that generate absorption bands in the UV and visible spectrum that deteriorate the optical fiber transmission in that spectral region. Lightguides containing an SiON core and SiO2—F cladding would be useful for ultraviolet applications because of the absence of Germanium in the lightguide core. SiON cores are useful in that they provide the ability to easily change the refractive index during manufacture within a 1.5-2.0 range by modifying the ratio of O2 to N2 in the glass matrix. This enables modification of the numerical aperture of SiO2/SiON and SiO2—F/SiON lightguides within a wide range. Furthermore, lightguides with SiON cores have a high radiation resistance. It would be extremely useful to have a method for production of such lightguides.
Presently, the most popular methods for manufacturing optical fiber preforms are Modified Chemical Vapor Deposition (MCVD), Outside Vapor Deposition (OVD), Vapor Axial Deposition (VAD), Plasma Enhanced CVD (PCVD), and Surface Plasma CVD (SPCVD).
The main disadvantage of MCVD, OVD, and VAD technologies for manufacturing preforms with active cores is that these methods require a step during the deposition in which soot glass layers are formed. The presence of soot makes it difficult to fabricate cores or claddings with steep doping profiles. Another disadvantage of these methods is the difficulty of achieving homogeneous doping, especially for lightguides with high rare earth element (REE) concentrations. Both the ability to deposit glass directly from the gas phase without a soot-forming stage and the ability to reduce the length of the active part of optical fiber lasers and amplifiers at high REE concentrations are the main advantages of deposition with a microwave plasma at low pressure. Additionally, only low pressure microwave plasma techniques have been able to provide effective N2-dissociation (when the electron temperature is essentially higher than gaseous one, or when Te>>Tg) because of the high bond energy of N2. Despite the advantages of plasma-chemical technologies in optical fiber preform production, effective tubeless microwave plasma-chemical technologies for the production of optical fiber preforms, including for example, activated and N2-doped preforms, have not been developed before the present time.
Methods for manufacturing dielectric rods by low pressure microwave plasma deposition are known. One such method is described in European Patent No. 0094053B1 by Beerwald et al. This method is also described in U.S. Pat. No. 4,508,554 by Beerwald et al, which is in the same family as European Patent No. 0094053B1. However, this method possesses three major drawbacks.
The first major disadvantage of the Beerwald patent is its inability to produce uniform microwave power density both in the microwave discharge and on the rod face using only either an E011 excitation mode or a circularly polarized H excitation mode. This results in a nonuniform deposition of doped silica glass on the rod face. Beerwald also provides for rotation of the rod, but rotating the rod is insufficient to fully compensate for the nonuniform deposition.
The second major disadvantage is that the Beerwald invention is capable of delivering only relatively low microwave power densities on the rod face when the disclosed operating parameters are implemented. For example, only 50-60 W/cm2 was achieved at the rod face when deposition on a described 60 mm diameter rod was attempted using the described 2 kW generator power under an operating gas pressure of 1 torr. The result of this low power density is a relatively low silica glass deposition rate (0.1-0.2 g/min) because the E011 resonator used by Beerwald is ineffective (it has low Q-quality in case of microwave plasma ignition).
The third major disadvantage is the Beerwald invention's inability to simultaneously provide both a temperature as high as 1200° C. and a high uniform power density (220-250 W/cm2). A high temperature is needed for effective removal of Cl2 from the deposited glass, and high uniform power density is required to produce a high deposition rate and effective dissociation and ionization of reagents at the rod face and in the microwave discharge. This invention is unable to perform these functions using only a single channel for the control and transmission of continuous microwave energy and thus one excitation mode.
An additional drawback to the Beerwald invention arises from the configuration of the described device. According to Beerwald, precursor gas is introduced above the substrate rod face. This configuration could lead to potential impurities or nonuniformities in the resultant deposition. Because the gravitational force tends to pull introduced gas particles toward the face of the substrate rod, the potential exists for unwanted particles, such as sootlike particles including Si, SiO2 and SiO, to drop onto the face of the substrate rod and reduce the purity of the preform produced. Additionally, the additional gravitational force on molecules created in the plasma can reduce control over the deposition of the molecules, and result in less effective control over the thickness of the deposition.